1. Field of the Invention
This invention relates to switch mode power supplies and more particularly to a start-up circuit for the pulse width modulated oscillator of such a power supply.
2. Prior Art
The fly-back type, isolated, switch mode power supply is often used to power electronic systems from an alternating current line because of its relatively low cost and ease of controllability. Essentially, such a system includes a rectifier for power derived from the alternating current source, a transformer, and an oscillating bi-stable switching circuit connected in series with the transformer primary and the rectified source. The oscillator chops the rectified source to provide power to the secondary of the transformer which powers the load. The oscillator rate can readily be controlled by feed-back from the transformer secondary and load to adjust the voltage and current provided to the load.
The bi-stable oscillator and its control circuit typically require a lower voltage power source than is supplied by the line rectifier. After the oscillator is initiated this lower voltage may be derived from a secondary winding on the transformer but means must be provided for initiating oscillator action before power is available at the transformer secondary.
Previous fly-back type, isolated, switch mode power supplies have used a variety of forms of start-up circuits for the oscillator but each of these circuits has exhibited certain disadvantages in terms of cost or power dissipation. The obvious but most expensive solution provides a separate low voltage power supply, connected to the line, for the oscillator. Another form of prior art start-up circuit employs a Zener regulator or dropping resistor connected across the direct current power source, but this arrangement dissipates substantial power if left on continuously and a circuit for disconnecting the low voltage power supply after the oscillating action is initiated would be relatively expensive due to the nature of a switch for the high voltage of the DC source.
It has alternatively been proposed to provide a capacitor in series with the direct current source and transformer primary but a relatively expensive switching circuit must be provided to discharge the capacitor to allow start-up after a short turn-off of the power supply.
The present invention is therefore directed toward a start-up circuit for the oscillator of a switch mode power supply employing low voltage, low cost components which automatically starts the circuit at any time and is switched off automatically as soon as the oscillating action starts.